The present invention relates to image processing systems and methods having a capability for processing halftone images to retrieve continuous tone (contone) images and more particularly to systems and methods for converting halftone images without screen structure and other halftone images to contone images.
Contone images are converted to halftone or binary images to enable the production of printed copies. In binary form, the image has pixels which are either black or white and which are printed by applying ink or not applying ink. It is usually difficult to process a halftone image to achieve, for example, scaling, enhancement or contrast change. Often, moire or distortion is introduced when a halftone image is directly processed.
The halftone image can be photographed with high contrast film to sharpen it, but details are lost. Alternatively, an out-of-focus copy can be made of the halftone image to blur out the halftones, but the resultant image looks blurry. If the halftone image is rescreened at a different frequency, the rescreened image typically exhibits moire patterns.
Normally, therefore, the halftone image should be first reconverted to a contone image to remove the old halftone screen and enable image processing. Thereafter, the contone image may be reconverted to a halftone image for printing. However, the quality of prior art unscreening has been limited and had limited the quality of the reconverted halftone and subsequent prints.
Image processing systems used with printers in reprographic systems typically require a capability for converting halftone images to contone images to meet reconversion needs and for converting scanned halftone images to contone images that can then be processed by any of a large variety of enhancement algorithms commonly available for contone images.
The halftoning process loses some image information in the conversion of the original contone image to a halftone image. The reconversion of a halftone image to a contone image accordingly is essentially an estimation process since the halftoning process cannot be reversed exactly to reproduce a contone image identical to the original image.
One common process for converting contone images to halftone images is a process called ordered dithering. The majority of images currently processed in the printing industry are dithered images since most printers can only print binary images. Generally, ordered dithering is a process in which a scanned continuous signal from a contone image is converted to a series of black (1 or ink) or white (0 or no ink) pixels with the pixel values determined by the pattern of a threshold or dither matrix to which the scanned signal is compared.
Another process used to convert contone images to halftone images is called error diffusion. No special thresholding matrix is used in the error diffusion process. Instead, a single threshold is applied to the whole image. Generally, image pixels are processed sequentially, i.e., the first pixel is made either 1 or 0 according to whether its gray level is above or below a predetermined threshold value such as 0.5. The first pixel error is then carried forward and added to the gray value of the unprocessed surrounding pixels in determining whether these pixels are above or below the threshold value. The resultant errors are then carried forward, and the process is continued until the image is completely processed. Error diffused images normally can only be printed with very good control over individual printed pixels.
The classic prior art method for converting halftone images to contone images, i.e., for "unscreening" contone images from halftone images, applies a spatial low-pass filter to remove the screen from the halftone image. The low-pass filter method typically blurs image edges or at least loses fidelity of edge information (fine detail) as a result of the fact that such image detail has spatial frequencies higher than those of the halftone screen. Overall, the low-pass filter removes the halftone screen but leaves a blurred and apparently defocussed image.
U.S. Pat. No. 4,630,125 to Roetling, and assigned to the present assignee, discloses a method for reconstructing a contone image of grayscale values that had been converted to a halftone image of black and white spots with the use of a known screen. The reconstruction method involves isolation of each spot of a halftone image along with a neighborhood of surrounding spots, and, for each neighborhood, comparing a maximum screen pattern value producing a white spot with a minimum screen value producing a black spot.
If the minimum screen value giving a black spot is greater than the maximum screen value giving a white spot, then the grayscale pixel value of the isolated spot is the average of the maximum and minimum screen values Just described. If the minimum screen value giving a black spot is less than the maximum screen value giving a white spot, then the process is repeated after deleting that portion of the neighborhood of surrounding spots containing the maximum or minimum screen value furthest from the isolated spot. Use of the Roetling scheme is limited to orthographic or digitally created and stored dithered images since it is based on the regularity of dots in a halftone image created with a dither.
Another U.S. Pat. No. 4,841,377 issued to Hiratsuka et al. discloses a method for estimating an original contone image from a stored binary image. The method involves, inter alia, setting a plurality of scanning apertures in a binary image formed of a dither matrix, selecting one scanning aperture satisfying a predetermined condition for each picture element of a continuous image to be estimated, and estimating the continuous image on the basis of the number of white or black picture elements in the scanning aperture selected. The Hiratsuka method is similarly limited to dithered halftone images.
More recently, U.S. Pat. No. 5,027,078, issued to Z. Fan, discloses a method for converting halftone images to contone images. The Fan method includes ways to estimate an improvement over the Roetling method through the application of "logic filtering." The logic-filter method provides best results for dithered halftone images that are scanned with high resolution, but it is also limited to dithered halftone images with a known or unknown but estimated screen,
Additional prior art that has limited relevance to the present invention follows:
1. U.S. Pat. No. 4,761,819, ADAPTIVE NOISE REDUCTION FILTER FOR RECONSTRUCTED IMAGES, dated Aug. 2, 1988, filed by Kenneth S. Denison, et al.
2. U.S. Pat. No 4,783,840, METHOD FOR ENHANCING IMAGE DATA BY NOISE REDUCTION OR SHARPENING, dated Nov. 8, 1988, filed by Woo-jin Song.
3. U.S. Pat. No. 5,038,388, METHOD FOR ADAPTIVELY SHARPENING ELECTRONIC IMAGES, dated Aug. 6, 1991, filed by Woo-Jin Song.
4. U.S. Pat. No. 4,811,239, "Digital Facsimile/Image Producing Apparatus", dated Mar. 7, 1989, filed by Sherman H. M. Tsao.
5. U.S. Pat. No. 4,561,239, IMAGE PROCESSING METHOD BASED ON PROCESSING OF INTERRELATED IMAGE GRADIENTS, dated Dec. 24, 1985; filed by Bryce E. Bayer.
In summary, the prior art generally has had shortcomings in preserving edge smoothing and avoiding edge blurring in the "unscreening" of contone images from halftone images. Further, the prior art generally has had no effective capability for converting halftone images created by error diffusion to contone images.